Many cam driven piston type pumps use tappet assemblies for pumping fluid. A tappet assembly includes a cam roller that is rotatably mounted on a roller pin. Typically, the cam roller follows an outer periphery of a rotating eccentric camshaft of the pump. The eccentric peripheral shape causes the cam roller to be actuated in a cyclical motion away from and towards the camshaft as it rotates. As the cam roller is cyclically actuated, the motion is translated to a plunger. During one portion of a pumping cycle, fluid is drawn by the plunger movement into a pumping chamber. During the second portion of the pumping cycle, the plunger pumps the fluid out of the chamber. Such tappet assemblies are known in the art.
In pumps such as high pressure pumps, the cam roller is exposed to extremely high loading. The high loading on the cam roller imposes a bending moment on the roller pin carrying the cam roller, thereby causing the roller pin to bend. Specifically, the roller pin bends over the entire length of the interface between the cam roller and the roller pin. The cam roller, however, does not bend at the edges of the interface between the cam roller and the roller pin because there is no bending moment at the roller ends. The bending of the roller pin without corresponding bending of the cam roller creates an imperfect and non-uniform contact area between the cam roller and roller pin. As a result, non-uniform contact pressure is applied at the edges of the contact area. Near the edges of the contact area, the contact pressure can be extremely high, as illustrated in FIG. 12, which is a graph of the contact pressure along the axial length of cam roller/roller pin interface of cam rollers and roller pins of the current art. This abnormal contact pressure near the edges of the contact area causes the cam roller to be overloaded and causes localized damage and wear to the inside diameter of the cam roller and outside diameter of the roller pin.
Current designs often require a “run-in” period during which the pump is ran to control the roller pin initial wear rate to reduce the non-uniformity between the roller pin and cam roller, and thereby reduce high end load pressures. To further reduce the effect of this problem and reduce the length of the run-in period, roller pins have been manufactured from softer metals such as bronze. The use of softer metals reduces the amount of run-in required because of the increased local wear rate and lower strength of the softer metals. Under the high contact pressure near the ends of the cam roller, the use of softer metals results in an increased rate of wear on the roller pin, thereby reducing the overall operating life of the tappet assembly before repair is required.